Energy storage capacitor

ABSTRACT

The capacitor includes at least one pair of electrodes separated by high-energy storage material consisting essentially of 85-95 percent by volume of an antiferroelectric ceramic and 5-15 percent by volume of nonreactive glass fired into a coherent body.

United States Patent Burn [451 Jan. 25, 1972 [54] ENERGY STORAGECAPACITOR [72] I Inventor: Ian Burn, Williamstown, Mass.

[73] Assignee: Sprague Electric Company, North Adams,

Mass.

[22] Filed: May 14, 1970 [211 App]. No.2 37,082

521 u.s.c| ..3l7/258,252/63.2 [s11 lnt.Cl. ..H0lgl/0l 5s]- FieldofSearch..252/63.2;317/258 [56] References Cited UNITED STATES PATENTS 2,293,07712/1966 Kaiser ..3 17/258 X 3,495,996 2/1970 Delaney ..3l7/258X PrimaryExaminer-E. A. Goldberg Attorney-Connolly and Hutz, Vincent H. Sweeney,James Paul OSullivan and David R. Thornton [57] ABSTRACT The capacitorincludes at least one pair of electrodes separated by high-energystorage material consisting essentially of 85-95 percent by volume of anantiferroelectric ceramic and 5-15 percent by volume of nonreactiveglass fired into a coherent body.

5 Claims, 4 Drawing Figures This invention pertains to energy storagedevices and more particularly to energy storage capacitors employingantiferroelectric materials.

In the prior art antiferroelectric materials which exhibit doublehysteresis loops and may be forced into a ferroelectric state by theapplication of an electric field above a certain threshold value, havebeen suggested for energy storage devices. The threshold value dependsupon both the material and temperature, and should be high atconventional operating temperatures in order to obtain worthwhile energydensities. However, antiferroelectric materials having high thresholdvalue and sufficiently high breakdown strength are not available in theprior art.

That is, the breakdown strength of the material must exceed thethreshold value in order for the ferroelectric state to be reached. Inthe prior art, this has only been achieved by compositions of lowthreshold value or by operation at exceptionally high temperatures whichlowers the threshold value below the breakdown point. Consequently,these result in low energy storage and objectionable temperaturerequirements.

SUMMARY OF THE INVENTION It is the object of this invention to provide ahigh energy storage capacitor suitable for conventional temperatureoperation. 1

It is another object of this invention to provide an antiferroelectricenergy storage material having high energy storage capacity and highbreakdown strength.

It is a further object of this inventionto provide an energy storagematerial of high breakdown strength which is a mixture ofantiferroelectric material and glass.

Broadly, an energy storage capacitor provided in accordance with theinvention comprises at least one pair of electrodes separated by highenergy material consisting essentially of 85-95 percent by volume ofantiferroelectric ceramic material selected from the group consisting oflead zirconate, lead hafnate and mixtures thereof and a -5 percent byvolume of glass which wets the ceramic material but produces minimumdissolution of it.

" BRIEF DESCRIPTION or THE DRAWING FIG. 1 is a perspective view partlybroken of a buildup of alternate layers of energy storage material andstaggered electrodes;

FIG. 2 is a perspective view of the energy storage capacitor chip cutfrom the structure of FIG. 1;

FIG. 3 is a perspective view of the chip of FIG. 2 with a dielectriccoating over the cooperating electrodes; and

FIG. 4 is a perspective view of the completed capacitor with leadsattached.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the preferred embodiments,high energy storage material which is a mixture of antiferroelectricceramic material and glass is interposed between two conductiveelectrodes. Preferably, the material is a mixture of substantially leadzirconate (PbZrO or lead hafnate (PbHfO in a glassy material, employedas a thin layer between metal electrodes of gold, silver, or silverplatinum alloys, or the like.

The high enbrgy storage material is formed by suspending particles of anantiferroelectric material in a glassy binder material which will wetthe ceramic particles with minimum dissolution thereof and bond them ina coherent body having high breakdown strength. Additionally, thetemperature coefficient of expansion of both ceramic and glass should beapproximately matched. A glass such as taught in U.S. Pat. applicationSer. No. 767,046, filed Sept. 26, 1968 by Galeb H. Maher will besuitable.

The method of forming the novel composition involves combining the hightemperature antiferroelectric ceramic powder with a low temperatureglass, and firing the combination at a temperature above the softeningpoint of the low temperature binder material. The low softening pointglass consists essentially of:

CdO 20-40% AID u-z t Bi,0, 13-32% 3,0, "-3 1. PhD l6-35% sio, 0-8% ZnO04.5% CaO 04% and preferably essentially consists of:

CdO 36% mp, 1% sec, 7 23% s,o so PbO 25% so, 5%

ZnO 5% Advantageously, the indicated glass does not excessively reactwith the antiferroelectric material so as to produce new crystallinephases in the composite body. Other types similar to the preferredceramic may be useful; however, glasses such as PbO-SiO -B,0 or BaO-A1 O-B O which produce excessive chemical interaction with theantiferroelectric material are unsuitable since they result incompositions having poor dielectric strength and low energy storage.

Referring to the drawing, FIG. 2 shows a buildup 10 of alternate layersof antiferroelectric material 12 and staggered conductive electrodes 14and 16 which may be of silver, gold, platinum or silver-platinum alloysor the like. The buildup 10 is cut or diced to form individual capacitorsections or chips 22 as shown in FIG. 2.

The individual chips 22 are coated with a dielectric material 24, forexample ceramic or the like, on the central portion thereof to cover theedges of the cooperating electrodes exposed by the dicing, as shown inFIG. 3. The units are then fired to give optimum densification, forexample, at approximately l,825 F. for 5 mindtes. Thereafter theelectrode ends are exposed at the end faces of section 22 by grinding orpolishing. A conductive fusible material 26 (such as a silver paste orthe like) is then applied to afford contact between" similar electrodes,and terminal leads 28 are attached to contacts 26 by soft solder or thelike.

In a specific example, a high'energy storage capacitor was formed in thefollowing manner:

grams of lead zirconate sold by Tamco under the trade name Ticon P2 wasmixed and milled with l3.l grams of the preferred low softening pointglass and 75 grams of ethyl cellulose binder in a porcelain mill until aparticle size of less than 5 microns was obtained.

A first electrode of a 60% Ag-40% Pd mixture approximately 0.2 milsthick was then screened on the upper surface of the formed layers withthe electrode extended to one longitudinal edge of the unit. Anapproximately 1.5-2.0 mil thick layer of the dielectric mixture wasscreened over the first electrode to form a first dielectric layer, andthen a second 2-mil thick Ag- Pd electrode as above was screened on theupper surface and extended to the opposite longitudinal edge of theunit.

Successive dielectric layers and alternating first and second electrodeswere then screened until several electrode pairs were completed. Thenseveral layers of the dielectric mixture were screened on the finalsecond electrode to form an approximately 10 mil thick top surface.

The unit was then diced into individual capacitor sections approximatelyfive-sixteenths inch by five-sixteenths inch, by transverse cuts whichleft alternate electrodes extended to the end faces. The sections wereremoved from the glass substrate and coated, except for their end faces,to a thickness of 2.0 mils with the dielectric mixture. Next, thesections were fired at I ,825 F. for approximately 5 minutes to giveoptimum densification. The end faces of each section were then polishedto expose and clean the electrode edges, and a silver paste deposited onthe end faces.

means of a Sawyer-tower circuit and an oscilloscope for AC fields over750 volts per mil (-peak.)

The energy storage of the sections when charged by DC fields up to 1,000volts per mil (l,500-2,000 volts DC) produced up to 35 joules per cubicinch.

Advantageously, the purity of the antiferroelectric material is notparticularly critical; some additives or impurities will increase theenergy storage density while others diminish it. Where additives areused in the antiferroelectric ceramic, the lead zirconate, lead hafnateor mixtures thereof should make up at least 50 percent by volume andpreferably over 90 percent by volume of the ceramic. As indicatedpreviously, the antiferroelectric ceramic should be 85 to 95 percent byvolume of the coherent ceramic glass body.

What is claimed is:

1. An energy storage capacitor comprising a high energy storage materialseparating at least a pair of conductive electrodes; said materialconsisting essentially of 8595 percent by volume of an antiferroelectricmaterial selected from the group consisting of lead zirconate, leadhafnate and mixtures thereof, and 15-5 percent by volume of glasscapable of wetting said antiferroelectric material with minimumdissolution thereof, said glass consisting essentially of CdO 20-40%Alp, o-z'r ago, 8-32% 5.0, 0-1 PbO 16-35% sio, u-s'r ZnO 04.5% CaO 0-515v 2. The capacitor of claim 1 wherein said high temperature ceramicmaterial is lead zirconate. 3. The capacitor of claim 1 wherein saidhigh temperature material is lead hafnate. v

4. The capacitor of claim 1 wherein said glass consists essentially of:

Cd() 36% A1 0, l% Bi,O 23% B 0; 5% PhD 25% SiO, 5%

ZnO 5% 5. The capacitor of claim 1 wherein said electrodes are selectedfrom the group consisting of silver, gold, platinum and silver-platinumalloys.

2. The capacitor of claim 1 wherein said high temperature ceramicmaterial is lead zirconate.
 3. The capacitor of claim 1 wherein saidhigh temperature material is lead hafnate.
 4. The capacitor of claim 1wherein said glass consists essentially of: CdO 36% Al2O3 1% Bi2O3 23%B2O3 5% PbO 25% SiO2 5% ZnO 5%
 5. The capacitor of claim 1 wherein saidelectrodes are selected from the group consisting of silver, gold,platinum and silver-platinum alloys.